The present invention relates to a laser driving apparatus suitable for use in an optical disk apparatus.
An optical disk apparatus for reading/writing a rewritable optical disk (such as a CD-RW and a DVD-RAM) needs to change a laser power level among three levels. These levels, from lowest level to highest level, are called a “read level”, a “erase level”, and a “write level”, respectively. The read level is used for reading an optical disk. The erase level is used for erasing information therefrom. The write level is used for writing information thereon. More particularly, laser power at the erase level causes the transition of the state of a surface of an optical disk to a pit non-formed state. Laser power at the write level causes the transition of the state of a surface of an optical disk to a pit formed state.
FIG. 1 shows an example of a laser power control circuit for controlling laser power in such a manner.
In the figure, reference numeral 2 designates a write level current source, reference numeral 4 denotes an erase level current source, and reference numeral 6 denotes a read level current source. The current sources 2, 4, and 6 respectively supply an electric current capable of generating laser power at the write level, an electric current capable of generating laser power at the erase level, and an electric current capable of generating laser power at the read level to a laser diode 30 through switches 8, 10, and 12. Reference numeral 28 denotes a photo detector for monitoring the laser power of the laser diode 30. Reference numeral 26 denotes an amplifier for amplifying a result of this monitoring.
Reference numeral 20 denotes a recording current control portion, reference numeral 22 denotes an erase current control portion, and reference numeral 24 denotes a reproducing current control portion. Each of the control portions 20, 22, and 24 performs a feedback control operation on a corresponding one of the current sources 2, 4, and 6 so that a corresponding generated laser power level comes close to a corresponding one of the predetermined levels. Reference numeral 34 denotes a recording data pulse generating portion that outputs pulse signals for turning on and off the switches 8 and 10 through switches 14 and 16. Reference numeral 32 denotes a recording-mode/reproducing-mode switch portion that outputs a mode switching signal Smode according to an operation mode of the optical disk apparatus. This mode switching signal Smode represents “1” when the operation mode becomes a recording mode, and represents “0” when the operation mode becomes a reproducing mode.
The on/off control of the switch 12 is performed according to the mode switching signal Smode inverted through the inverter 18. In the reproducing mode, the switch 12 is set to an on-state. In the recording mode, the switch 12 is set to an off-state. The on/off control of the switches 14 and 16 is performed according to the mode switching signal Smode. In the reproducing mode, the switches 14 and 16 are set to an off-state. In the recording mode, the switches 14 and 16 are set to an on-state. Therefore, in the recording mode, the recording data pulse generating portion 34 is enabled to perform the on/off control of the switches 8 and 10 through the switches 14 and 16.
Next, the waveform of a laser diode output current obtained according to the aforementioned configuration is described with reference to FIGS. 2A to 2F. At a moment t1 shown in FIG. 2A, the mode switching signal Smode rises from “0” to “1”. That is, the operation mode is changed from the reproducing mode to the recording mode. Incidentally, the “recording mode” is further classified into a “data recording mode”, in which readable data is recorded on an optical disk, and a “DC erase mode”, in which the already recorded data is simply erased.
In the data recording mode, a recording EFM signal generated as shown in FIG. 2B is supplied to the recording data pulse generating portion 34 from a control circuit (not shown). In the recording data pulse generating portion 34, the switching control of the switches 8 and 10 is performed so that a recording current and an erase current respectively shown in FIGS. 2C and 2D are outputted correspondingly to the recording EFM signal through the switches 8 and 10. Incidentally, the recording current corresponds mainly to the level “1” of the recording EFM signal, while the erase current corresponds mainly to the level “0” of the recording EFM signal. The generated recording current and the generated erase current are super imposed. Then, a resultant signal is supplied to the laser diode 30 as a laser diode output current shown in FIG. 2E.
Incidentally, the recording current is modulated by using the write level Pw as a peak value so that the waveform thereof has a pectinate shape. This prevents a pit from being deformed like a teardrop owing to a heat accumulation effect. The formation of the waveform of a recording current in this manner is called “a strategy process”. The strategy process enables the formation of various waveforms, such as a waveform, in which the width of a leading pulse is set to be wider than that of a subsequent pulse, another waveform whose duty ratio is changed, and another waveform whose rising timing and falling timing are changed. Meanwhile, in the DC erase mode, no recording current is outputted. Further, only an erase current is continuously outputted. Consequently, the laser diode output current is continuously at the erase level Pe, as shown in FIG. 2F.
Meanwhile, it is frequent that a high frequency signal is superimposed on an output current to the laser diode 30. This is to reduce an optical feedback noise and to accurately detect a wobble signal. Various kinds of manners of high-frequency superposition are illustrated in dashed line regions A, B, and C in FIG. 1. First, in the dashed line region A, reference numeral 36 denotes a high-frequency oscillator that is connected to an output terminal of the read level current source 6 and that superposes a high-frequency signal on a read level current.
Another manner of performing high-frequency superposition is illustrated in the dashed line region B. In the figure, reference numeral 40 denotes a switch that selects one of a high-frequency signal outputted from the high-frequency oscillator 36, and another high-frequency signal obtained by amplifying the high-frequency signal outputted from the oscillator 36 through an amplifier 38, according to the mode switching signal Smode and that superposes the selected signal on a laser output current. A switch 40 is switched according to an operation mode. That is, during the recording mode, a contact provided at the side of the amplifier 38 is selected. During the reproducing mode, a contact provided at the side of the high-frequency oscillator 36 is selected. In the case of this manner of performing high-frequency superposition, during the recordingmode, a high-frequency signal is superposed on both the currents that are at the erase level and the write level.
Another manner of performing high-frequency superposition is illustrated in the dashed line region C. This manner similar to the manner illustrated in the dashed line region A in that a high-frequency signal outputted from the high-frequency oscillator 36 shown in the figure is superposed onto the read level current. However, this high-frequency signal is amplified through an amplifier 38. Then, the amplified high-frequency signal is superposed on an erase level current.
Hereunder, the techniques of performing high-frequency superposition by utilizing the manners respectively illustrated in the dashed line regions A, B, and C are referred to as “unit A, unit B, and unit C”. Incidentally, the reason for superposing the amplified high-frequency signal on the erase level current or the write level current in the unit B and the unit C is that the erase level and the write level are considerably high, as compared with the read level, and that thus, a sufficient effect cannot be obtained unless the signal level of the high-frequency signal is raised according to the erase or write level. FIGS. 3A to 3C illustrate laser diode output currents, on each of which a high-frequency signal is superposed, to be respectively provided during the data recording and during the DC erase according to the unit A, the unit B, and the unit C.
However, according to the unit A, high-frequency superposition is performed only in the recording mode. Thus, as illustrated in FIG. 3A, the high-frequency superposition is not performed at all during the DC erase. Therefore, during the DC erase, there are caused drawbacks that the optical feedback noise increases, that the quality of the wobble signal is degraded, and that thus a spindle control operation is not accurately performed. Such drawbacks are eliminated by the unit B and the unit C, according to each of which high-frequency superposition is performed even during the erase. However, there is the need for amplifying the high-frequency signal. Consequently, it is necessary to provide the amplifier 38 in the circuit. This results in increase in the number of components of the circuit and in the manufacturing cost thereof. Furthermore, in the case of the unit B according to which the high-frequency superposition is performed on the write level current, a high-frequency signal is superposed on a current that is at a top power level. Thus, the circuit should use a high-rating (expensive) laser diode so as to prevent the laser diode 30 from being broken. Consequently, the unit B has a drawback in that the use of the high-rating laser diode results in further increase in the cost of the circuit.